A
Limiting Factor is the available
amount of the scarcest resource
necessary for life.

Here we come to a
very important
idea. It is
the idea of limiting
factors.

Every organism needs
resources to draw
on, for example, an
energy source
(food), water,
special elements in
nutrients, oxygen or
carbon dioxide, a
comfortable temperature range,
shelter, and so
on. If the
organism does not
have access to a particular
essential resource, it will
stop growing,
struggle, and die.
The
abundance or
scarcity of the
resource is a limiting
factor in terms
of the number of a
type of organism
that can live in a
specific
environment.

Let's think of this
in terms of
examples, which will
make this idea easy
to understand.

Why
are there no plants
on this hill?

This hill is located
in Death Valley, a
desert area in
California. The
limiting factor here
is the availability
of water.

Infrequent rains
wash down these
steep slopes and
flow away in a
short lived
stream. Such
water as soaks into
the earth is quickly
surrendered to the
hot sun and
evaporates away.

Why
do people and air
breathing animals drown?

When
people sink into
water, they
drown.
Why? There is
a limiting factor --
oxygen.
Our lungs cannot
absorb oxygen from
water, nor get rid
of the carbon
dioxide in our blood
in it.
Air-breathing
organisms
can live only a very
few minutes without
oxygen.

Why
are there no lizards
in polar regions?

The
bodies of cold
blooded (exothermic)
animals are the same
temperature as their
environment.
In the hot deserts
the snakes and
lizards move slowly
in the morning and
lie out in the sun
so that their bodies
and their internal
chemistry can warm
up. The
limiting factor here
is temperature.
I don't know if a
lizard in Antarctica
would ever get warm
enough to feel
hungry or to be able
to move enough to
find or capture
food. There
are very few insects
to eat there anyway.

Why
are there no sheep
in Antarctica?

Well,
sheep are
warm-blooded (endotherms)
but they are
grazers, and the
Antarctic is too
cold to grow
grass. (Well,
there are a few
sprigs of one kind
of grass that does
grow on the exposed
earth there.)
Also almost all of
the land is covered
by ice and snow, so
there is no
available soil to
provide nutrients
for the
grass. The
sheep would freeze
to death in
Antarctica anyway
(limiting
factor: their bodies
cannot deal with
this very cold
climate) and would
also starve if they
lived long
enough. The
grass also has two
problems -- low
temperatures, and water
that cannot be
absorbed when
frozen.

Seasonal
Resources

Limiting
factors can be
seasonal. In
much of the world
the winter is too
cold to support
plant growth, but
the return of spring
brings warmer
temperatures and
lush
vegetation.

In
deserts, if there
has been rain during
the cooler months,
there may be a
brief but lavish
display of flowers
in the
spring.
It is important for
desert plants to
rush to the seed
making stage before
the water disappears
again. Here
you can see a plant
that "made
it". The
dry, empty seed pods
tell us of its
success.

Light
also varies from
season to
season. In
polar regions the
plants, warmed
somewhat by the
long, sunny days,
photosynthesize and
grow rapidly in
summer. They
have evolved to grow
rapidly from seed to
plant to flower to
seed again in the
short gtowing
season. The
limiting factor here
is time. There
is only a short
period during which
all the other
important factors
are available.

Even the
harvest of the
seasons can be
affected by such
events as extensive
forest fires
or large
volcanic
eruptions. If
enough fine ash
makes its way into
the atmosphere, it
can block out some
of the sun's heat
and light, creating
colder than usual
weather. The
amount of light and
heat available
become the limiting
factors for plant
growth.

Invisible
Resources

Not
all requirements are
so obvious. It
makes sense that we
do not see lush
jungle growth in
deserts, where the
rainfall is
sometimes far less
than ten inches a
year. However,
there are other
limiting factors
that we may have to
think about a bit.

Factors like the
acidity of the soil
and the presence of
trace elements may
limit or prevent
plant growth.
In the ocean, algal
growth is limited by
the amounts of
available nitrogen,
phosphorus, or iron
dissolved in the
water.
Scientists have
experimented with
scattering iron
filings into the
ocean and have seen
rapid growth of the
seaweed and
phytoplankton.